Billionaire Mayor Mike Bloomberg is asking prominent Democratic donors in New York, a key source of funds for candidates across the country, to stop contributing to the four Democratic senators who voted to block a bill that would have strengthened background checks for gun buyers, the New York Times reported.

So it's worth looking at how reliant the senators are on New York funders. Donors from the Empire State have been responsible for 7.5 percent of the money funding the federal campaigns of Sens. Max Baucus, D-Mont., Heidi Heitkamp, D-N.D., Mark Pryor, D-Ark., and Mark Begich, D-Alaska. In all, New Yorkers gave $3.4 million to their campaigns, according to an analysis of data in Influence Explorer. Pryor and Begich are up for reelection in 2014, Baucus is retiring at the end of his term and Heitkamp does not face voters until 2018.

Of the four, Begich is the most reliant on New York money — at 10.5 percent, followed by Heitkamp (9.1 percent), Baucus (7.9 percent) and Pryor (4.6 percent).

Bloomberg blames the four senators for the defeat of the background check measure, which won the support from just three Republicans. The bill needed five more votes to clear a filibuster — meaning that even if the Democratic holdouts had all flipped their votes, at least one other Republican would have had to support it to move the measure to a floor vote.

Bloomberg's outside group, Mayor Against Illegal Guns, has already funded a $350,000 TV ad campaign against Pryor for his vote. The group has also been running ads in states all over the country criticizing other senators for their votes against the bill, praising others for their affirmative votes, and supporting gun control legislation in states like Nevada.